An article surveillance system has been developed wherein a transmitter or generator periodically derives a first inductive magnetic field having a carrier frequency. The first magnetic field is coupled to a structure on a surveilled article. The structure resembles a tuned or resistance-inductance-capacitance (RLC) circuit. Upon completion of the first magnetic field, the structure radiates a second magnetic field having a frequency that is equal to or approximately equal to the frequency of the first field. A receiver for the second magnetic field is activated in synchronism with the derivation of the second magnetic field. Preferably, the receiver is effectively disabled while the first magnetic field is being derived so that the first magnetic field is not detected as the field associated with the structure.
The receiver detects the occurrence of the second magnetic field for a predetermined time to indicate the presence of the surveilled article. In the preferred prior art receiver, the detection process involves synchronously demodulating a replica of the second magnetic field as incident on a pick up coil arrangement of the receiver. The length of time that a synchronous demodulator derives an output having a frequency equal to the frequency of the second magnetic field provides a measure of the length of time that the second magnetic field is incident on the coil arrangement. The length of time is determined by an integration process that is initiated immediately after expiration of the first magnetic field and has a predetermined duration determined by the band width of the detection process. While the first magnetic field is being derived, the synchronous demodulator and an integrator means are effectively decoupled from the receiver coil arrangement by reducing the gain of an amplifier between the coil and synchronous demodulator to zero. Thus, it is important to provide synchronization between the derivation of magnetic field pulses from the transmitter and operation of the receiver.
In the past, synchronization between the transmitter and receiver of article surveillance systems has been usually attained by physically linking the transmitter and receiver with a hard wire cable, activated by a periodic source, e.g. an oscillator which is a part of the transmitter. The oscillator causes the transmitter to supply periodic pulses to the receiver via the hard wire cable. The use of a hard wire cable between the transmitter and receiver results in a further installation expense of the article surveillance system because of the desire to hide any wires which extend through the region where the transmitter and receiver are located. Seemingly, the need for a cable between the transmitter and receiver can be obviated by utilizing electric and/or magnetic fields to synchronize the transmitter and receiver. However, the use of such fields is beset with pitfalls, particularly because of likely interference.
It is, accordingly, an object of the present invention to provide a new and improved article surveillance system including a synchronized transmitter and receiver.
A further object of the invention is to provide a new and improved article surveillance system wherein synchronization of a transmitter and receiver is attained without any added wires between the transmitter and receiver, as well as without the need for electric or magnetic fields between the transmitter and receiver.